Dual microphone system



- Dec. 13, 1949 w. B.- BRUENE DUAL MICROPHONE SYSTEM Filed Feb. 14, 1948DYNAMIC MICROPHONE I Em OUTPUT TO NEXT AMPLIFIER CARBON- MICROPHONEINVENTOR WARREN B. .BRUENE ATTORN Y Patented Dec. 13., 1949 UNITEDSTATES PATENT OFFICE DUAL MICROPHONE SYSTEM Warren B. Bruene, CedarRapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, acorporation of Iowa Application February 14, 1948, Serial No. 8,399

7 Claims.

This invention relates to microphone-amplifier systems and moreparticularly to such systems for use with microphones of differentcharacteristics.

A principal object of the invention is to provide a microphone-amplifiersystem which is capable of producing substantially uniform outputregardless of Whether the input is provided from a carbon microphone ora dynamic microphone.

Another object is to provide a microphone-amplifier system wherein theamplifier is arranged to be driven either by a dynamic microphone or acarbon microphone, together with switching and circuit arrangements forrendering the output of the amplifier substantially uniform for eitherof the types of microphones that is used.

A feature of the invention relates to an amplifier which is adapted tobe driven either by a dynamic microphone or a carbon microphone,together with adjustable circuit arrangements common to both microphoneinputs for producing substantially uniform output from the amplifierregardless of which microphone is being used.

A further feature relates to the novel organization, arrangement andrelative interconnection of parts which cooperate to provide an improveddual microphone amplifier system.

Other features and advantages not particularly enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims.

In certain types of communication apparatus, for example in voicefrequency amplifiers, modulators and the like, it is sometimes necessaryto derive the input signals either from a dynamic microphone or a carbonmicrophone. One of the drawbacks to the use of these two different typesof microphones is that they require different amplifier gains in orderto produce the same output from the amplifier. The dynamic typemicrophone requires a greater gain than does the carbon microphone, butthe carbon microphone requires a separate source of power such as abattery in order to render it operative.

There is shown in the drawing a microphone amplifier system wherein theamplifier may be driven by either type of microphone while at the sametime preserving substantial uniformity of output regardless of whichmicrophone is used. For this purpose, there is provided an amplifiertube l which. may be of any well-known type, and

while the drawing shows this tube as of the triocle indirectly heatedcathode type, it will be understood that any other well-known type ofamplifier tube may be employed. The dynamic microphone 2 is connectedacross the control grid 3 and the cathode E of tube i, a suitable highresistance grid leak 5 being provided across thegrid and cathode. Thedirect current supply for the plate or anode 6 is representedschematically in the drawing by the battery 7, the negative terminal ofwhich is grounded and the positive terminal of which is connected to theprimary winding 8 of an audio frequency coupling transformer 9 andthence to the plate 5. The secondary Winding ill of this transformer canthen be connected to any suitable additional amplifier stage as may berequired. The necessary bias between the grid 3 and cathode i isachieved by means oi'the usual cathode bias resistor I I connectedbetween the cathode i and ground, this bias resistor. ll being shuntedby the condenser l2, under control of the single-pole single-throwswitch i3, which is closed when the dynamic microphone 2 is being used.Under these conditions, the tube I acts in' the well-known manner toamplify the signals applied to the grid 3 from the dynamic microphone.

When it is desired to use the carbon microphone E, the switch I3 isopened. It will be observed that a part of the direct current from thepower source I flows to ground through the resistors l5, H3, in series,which therefore act as voltage dividers. The microphone I4 is connectedacross the resistor It to supply the necessary power for thatmicrophone. The sound waves incident upon the microphone l4 produceaudio frequency variations which are coupled through the adjustableresistor l l and thence through the condenser l2 to the cathode 4, thusdriving the cathode 4'. in accordance with these audio frequencysignals. Thus, the condenser l2 which normally would function as theby-pass condenser for resistor ll as above described, now functions as adirect current blocking condenser between the carbon microphone l4 andthe driven cathode 4. This method of driving the tube 1 by variation ofthe cathode potential 4, results in lower gain from the tube l. However,this is a desirable eifect in the system as disclosed, since normallythe carbon microphone l4 would produce a higher audio output at itsterminals for a given sound wave incident thereon, as compared with thesame sound wave incident upon the dynamic microphone. By adjusting theresistor I], it is possible to maintain the output from transformer 9substantially uniform regardless of which of the two microphones isbeing employed.

When the system is being used, it is not necessary to adjust theresistor I! for each transmission. It may be initially set to producethe same gain for a given intensity of sound upon each microphone andwhen once set, it can remain in its adjusted position. Consequently, allthat is necessary to switch from the dynamic microphone input to thecarbon microphone input is to open the switch IS. The condenser l2therefore performs the double function of acting as a by-pass condenserwhen the dynamic microphone is being used and as a direct currentblocking condenser when the carbon microphone is being used.

The expression dynamic microphone as employed in the specification andclaims means any microphone which does not require a separate battery orpower source to render it effective; and includes any of the well-knowntypes of microphones which generate a voltage in response to impingingsound waves, e. g., crystal microphones, ribbon microphones, movingcoil-magnet microphones, velocity microphones and the like.

While one particular embodiment has been described, it will beunderstood that various changes and modifications may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A microphone amplifier system, comprising a grid-controlled amplifiertube, a microphone of the dynamic type, another microphone of the typewhich requires a local source of direct current power, and atwo-position switch for operatively connecting either microphone to theinput of said amplifier while maintaining the amplifier outputsubstantially uniform for a given sound input regardless of whichmicrophone is being used, and circuit connections operative when saidswitch is closed in one position to connect said other microphone onlycapacitively to the cathode of said amplifier tube.

2. A microphone amplifier system, comprising a grid-controlled tube, amicrophone of the type which generates voltage in response to impingingsound waves, a carbon microphone which varies the resistance in anassociated direct current supply circuit, means comiecting thefirst-mentioned microphone to drive the control grid of said tube, meansincluding a series coupling condenser 4 connecting the second-mentionedmicrophone to drive the cathode of said tube, and switch means foroperatively connecting either of said microphones with said amplifier.

3. A microphone amplifier system, comprising a grid-controlled amplifiertube, a microphone of the dynamic type, a microphone of the nondynamictype, a cathode bias resistor for said tube, a by-pass condenser forsaid resistor, a two-position switch, circuit connections efiective withsaid switch in one position for operatively isolating the non-dynamicmicrophone from the amplifier and connecting said condenser across saidresistor, and other circuit connections effective when said switch is inanother position for operatively connecting said non-dynamic micro- 4phone to the cathode of said amplifier through said condenser whileremoving said condenser from across said resistor.

4. A microphone amplifier system, comprising a dynamic microphone, anon-dynamic microphone, a grid-controlled amplifier tube for bothmicrophones, said tube having a cathode bias resistor, means includinganother resistor and a condenser connecting said non-dynamic microphonein series between said cathode and ground, a two-position switch,circuit connections effective in one position of said switch to isolatesaid non-dynamic microphone from said cathode and simultaneouslyconnecting said condenser in shunt to said bias resistor and othercircuit connections effective when said switch is in another position todisconnect said condenser from said bias resistor.

5. A microphone amplifier system, comprising a dynamic microphone, anon-dynamic microphone, a grid-controlled amplifier tube for bothmicrophones, a switch for selectively rendering either of saidmicrophones effective on said amplifier, a cathode bias resistor andshunt condenser for said tube, means effective with the switch in oneposition to drive the grid of said tube by the output of said dynamicmicrophone, means effective with the switch in another position to drivethe cathode of said tube by the output of said non-dynamic microphone,said switch when in said other position disconnecting said condenserfrom across said bias resistor.

6. A microphone amplifier system, comprising a pair of microphones ofdifierent signal outputs, a grid-controlled amplifier tube for bothmicrophones, a bias resistor connected between the cathode of said tubeand ground, means connecting one microphone across the grid and ground,a condenser, a two-position switch effective in one position to bridgesaid bias resistor by said condenser and efiective in another positionto disconnect said condenser from across said resistor, and meansconnecting the other microphone effectively in series with saidcondenser and cathode when said switch is in said other position.

7. A microphone amplifier system, according to claim 6 in which anadjustable resistor is connected in series between said other microphoneand said condenser for equalizing the outputs of the amplifierregardless of which microphone is being used.

WARREN B. BRUENE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,109,756 Sparks Mar. 1, 19382,114,019 Friebus Apr. 12, 1938

